Sucker rod pumps are positive displacement pumps used to pump hydrocarbons and other liquids from wells. These pumps are located in the well bore below the liquid level of the liquid to be pumped. The pump has a barrel within which a piston slides up and down. The piston is moved by a rod which extends to the surface, where it is moved up and down by a surface-located pumping unit. The barrel has an inlet check valve at the bottom of the barrel and an outlet check valve in the piston. Fluids flow from the top of the barrel, via the outlet check valve, to a tubing which extends to the surface. When the piston slides down, fluids are forced through the outlet check valve in the piston, with the inlet check valve at the bottom of the barrel seating. When the piston is stroking upward, the check valve on the piston seals, and the fluids within the barrel above the piston are lifted upward, into the tubing which extends to the surface. At the same time, the piston draws fluids into the barrel through the inlet check valve. These pumps have proven to be simple and reliable although not without shortcomings. When fluids which are near their flash point temperature are being pumped, they will partially vaporize when they are drawn into the barrel. Any vapor which is drawn into the barrel must be compressed to the pressure of the discharge tubing before the top check valve will open. The volume of vapors within the pump can be great enough that the full stroke of the piston will not achieve a sufficient pressure to force the outlet check valve open. When this happens, the pump is in a state referred to as vapor locked. When the fluid being pumped has a considerable amount of dissolved light material, the pump will be subject to vapor locking. When an oil field is subject to a steam flood, producing wells will contain a mixture of oil and condensate near its flashing point. This fluid can also cause sucker rod pumps to vapor lock.
When a sucker rod pump is vapor locked, it is typically shut down for a period to break the vapor lock. During this period, vapors will have a chance to escape through check valves, and the pump can cool due to the absence of the heat of compression. The vapor lock will eventually break, and pumping can then be continued. Less patient operators adjust the length of the rod, allowing the piston to bang against the ends of the barrel. This causes the outlet and/or the inlet valve to unseat and break the vapor lock. Neither of these solutions to the problem of vapor locking sucker rod pumps is acceptable.
Pumps have been developed which are less prone to vapor locking, but these pumps each have shortcomings. One such pump is described in U.S. Pat. No. 4,221,551. This pump has two sliding valves, both within a barrel, and above a plunger. When the plunger is moving downward, both valves are in lower positions. In the lower position, the bottom valve (inlet valve) has ports aligned with ports in the barrel providing communication between the well borehole and the pump barrel. The top sliding valve (outlet valve) functions like a check valve, sliding upward when the barrel pressure exceeds the pressure at the bottom of the well string. When slid to its upper position, the top valve has ports which align with ports in the barrel to provide communication between the barrel and the well string. When the lower sliding valve (inlet valve) is moved to an upper position, the ports are not aligned. The lower valve is an inverted cup configuration with inlet ports or a sleeve, and shoulder ports on the top to allow fluids to communicate from the lower position of the barrel to the top sliding valve. The lower valve is moved to the upper position by the pressure differential created by flow being forced through shoulder ports. Therein lies the shortcoming of this design. There is no significant flow through these shoulder ports until the pressure within the barrel exceeds the pressure at the bottom of the well string, opening the outlet valve. The bottom valve will therefore not move until the top valve opens. Pressure within the barrel must build as a result of the rising plunger in spite of the inlet valve remaining open. This dictates that the inlet ports must have small flow areas because large ports would result in the plunger forcing flow in and out of the working barrel through the inlet ports without ever achieving a pressure sufficiently high to open the outlet valve. The small size of the inlet ports ensures that flashing will occur when fluids near their bubble points are pumped. The mechanism of closing the inlet valve ensures that a significant portion of the upward stroke of the pump will not be productive due to fluids exiting the barrel through the inlet valve ports. This design therefore has many shortcomings.
Another sucker rod pump design which is said to prevent vapor locking is described in U.S. Pat. No. 3,046,904. This design also has inlet and outlet valves above a plunger. This pump utilizes about 24 small ball check valves as inlet valves, all located in an inlet shroud around the top of the working barrel. The small inlet check valves of this design, again, assure that some flashing will occur when fluids near their bubble points are pumped. Further, if sufficient vapors get into the pump barrel, the pump will, in fact, vapor lock. There is no mechanism to release vapors from the barrel other than to compress them and force them up the well string. If the plunger stroke does not compress vapors within the barrel to the pressure of the bottom of the well string, a vapor lock will result. This design therefore does not solve the problem of preventing vapor lock of sucker rod pumps.
Another subsurface pump which is said to avoid vapor locking is described in U.S. Pat. No. 3,136,256. This pump avoids vapor lock by mechanically lifting a standing valve ball (intake valve) near the top of the intake stroke of the pump. This design has the inlet valve in the typical bottom position. What is vented is therefore the volume which is least likely to have a large amount of vapor. The timing of the opening of the valve is also undesirable. Opening the valve at the end of the inlet stroke almost ensures that the stroke will not result in fluids being pumped up the well string. Opening the valve at the end of the inlet stroke also vents the barrel at the lowest possible barrel pressure. At this pressure, because of the limited time available for venting the vapor locked barrel and then replacing the vapor with pumpable liquids, there will be very little driving force to expel compressed vapors. It is unlikely that venting the barrel at this point in the pumping stroke will even break a vapor lock because breaking the vapor lock not only requires that the excess pressure be vented, but that liquids flow into the barrel to be pumped on the downstroke. Both venting the barrel and filling a significant portion of the barrel with pumpable fluids is not likely to be achieved by merely opening the inlet valve at the top of the inlet stroke.
It is therefore an object of the present invention to provide a well pump which is capable of pumping volatile fluids and is resistant to vapor locking. It is a further object of the present invention to provide a deep well pump which is of a simple design with relatively few moving parts and relatively few sealing surfaces. It is a further object to provide a deep well pump having one or more inlet valves which has a very low pressure drop through the inlet valve. It is another object of this invention to provide a well pump which will not fluid pound.